1. Field of the Invention
The present invention relates to a projecting optical system, and more particularly to a projecting optical system for use in a multiple-panel image projection apparatus such as a liquid crystal projector.
2. Description of the Prior Art
Recently-developed liquid crystal projectors typically adopt a multiple-panel system regardless of whether they employ transmission-type or reflection-type display panels. In such a liquid crystal projector, color integration is commonly achieved by the use of a prism having a color-integrating thin film (i.e. a dichroic prism). As an example of an image projection apparatus having transmission-type display panels in which color integration is achieved by the use of a prism, Japanese Laid-Open Patent Application No. S63-116123 proposes a display apparatus. In this display apparatus, the light emitted from a light source is separated into light components of three different colors by two dichroic mirrors. The three light components are then individually passed through separate liquid crystal light valves, and are then, through color-integrating cross dichroic prism, directed into a projecting optical system.
As another example of an image projection apparatus having reflection-type display panels in which color integration is achieved by the use of a prism, Japanese Laid-Open Patent Application No. H3-249639 proposes a liquid crystal projector. In this liquid crystal projector, the light emitted from a light source is first reflected from a polarization beam splitter and separated into light components of three different colors by a cross dichroic prism. Then, the three light components are individually modulated by being reflected from separate display panels, and are then passed through the cross dichroic prism and the polarization beam splitter once again so as to be directed into a projecting optical system.
On the other hand, image projection apparatuses are known that achieve color integration without using a prism. For example, Japanese Laid-Open Patent Application No. H3-78738 proposes an image projection apparatus that has transmission-type display panels and achieves color integration by the use of a mirror. In this image projection apparatus, the light emitted from a light source is separated into light components of three different colors by two dichroic mirrors, and the three light components are then individually passed through separate transmission-type display panels. After being modulated by the transmission-type display panels, the light components of different colors are reflected from or transmitted through each of two successively arranged color-integrating dichroic mirrors, and are then directed into a projecting optical system.
Japanese Laid-Open Patent Application No. H8-334727 proposes an image projection apparatus that has reflection-type display panels and achieves color integration by the use of a mirror. This image projection apparatus has, like that proposed by Japanese Laid-Open Patent Application No. H3-78738, two successively arranged color-integrating dichroic mirrors that individually reflect or transmit light components of different colors. However, in this image projection apparatus, light is incident on the dichroic mirrors with an incident angle of 45.degree. or less, and the optical axes leading to the two dichroic mirrors are substantially perpendicular to each other (i.e., the plane including the optical path of the light incident on the display panels and the optical path of the light reflected therefrom with respect to one dichroic mirror is perpendicular to the corresponding plane of another dichroic mirror). U.S. Pat. No. 5,231,431 proposes, as does Japanese Laid-Open Patent Application No. H3-78738, an image projection apparatus that has transmission-type display panels and achieves color integration by the use of a mirror. However, this image projection apparatus has a cylindrical mirror for correcting astigmatic difference disposed between display panels and a projection lens system.
In general, a prism designed for color integration has a relatively large volume, and therefore a sufficiently large glass block is required to manufacture such a prism. Moreover, with such a prism, cementing needs to be carried out with extremely high accuracy. Accordingly, using a cross dichroic prism to achieve color integration, as proposed in Japanese Laid-Open Patent Applications Nos. S63-116123 and H3-249639, leads to an undesirable increase in manufacturing costs.
Moreover, using a mirror instead of a prism to achieve color integration, as proposed in Japanese Laid-Open Patent Application No. H3-78738, causes astigmatic difference in the light rays transmitted through the dichroic mirror, as in the light rays transmitted through a flat glass plate held at angle with the optical axis. This degrades the quality of the projected image. As proposed in Japanese Laid-Open Patent Application No. H8-334727, if two display panels are so disposed that the plane including the optical path of the incident light and the optical path of the reflected light with respect to one dichroic mirror is perpendicular to the corresponding plane of another dichroic mirror, the two dichroic mirrors act to cancel out each other's astigmatic difference, and thus it is possible to correct astigmatic difference. This, however, requires the display panels to be arranged in a complicated manner and thus makes it difficult to hold them in position or adjust them. In addition, the image projection apparatus as a whole needs to be made larger in height. Moreover, disposing a member for correcting astigmatic difference between display panels and a projection lens system, as disclosed in U.S. Pat. No. 5,231,431, requires an accordingly long back focal length, and thus makes it difficult to secure sufficiently high optical performance.